Variable bandwidth MRI data collection

ABSTRACT

The present invention is directed to an MRI apparatus. It includes a main magnet  12  that generates a substantially uniform temporally constant main magnetic field, B 0 , through an examination region  14  wherein an object being imaged is positioned. A magnetic gradient generator produces magnetic gradients in the main magnetic field across the examination region  14 . A transmission system includes an RF transmitter  24  that drives an RF coil  26  which is proximate to the examination region  14 . A sequence control  40  manipulates the magnetic gradient generator and the transmission system to produce an MRI pulse sequence, such as an FSE pulse sequence. The MRI pulse sequence induces magnetic resonance echos  66  from the object. A reception system includes a receiver  30  that receives and demodulates the echos 66 at varying sample rates and varying bandwidths. A reconstruction processor  50  reconstructs a single image from data collected via the reception system, and an output device produces a human viewable rendering of the image.

BACKGROUND OF THE INVENTION

The present invention relates to the art of medical diagnostic imaging.It finds particular application in conjunction with magnetic resonanceimaging (MRI), and will be described with particular reference thereto.However, it is to be appreciated that the present invention is alsoamenable to other like applications.

In MRI, a substantially uniform temporally constant main magnetic field,B₀, is generated within an examination region. The main magnetic fieldpolarizes the nuclear spin system of a subject being imaged within theexamination region. Magnetic resonance is excited in dipoles which alignwith the magnetic field by transmitting radio frequency (RF) excitationsignals into the examination region. Specifically, RF pulses transmittedvia an RF coil assembly tip the dipoles out of alignment with the mainmagnetic field and cause a macroscopic magnetic moment vector to precessaround an axis parallel to the main magnetic field. The precessingmagnetic moment, in turn, generates a corresponding RF magneticresonance signal as it relaxes and returns to its former state ofalignment with the main magnetic field. The RF magnetic resonance signalis received by the RF coil assembly, and from received signals, an imagerepresentation is reconstructed for display on a human viewable display.

The appropriate frequency for exciting resonance in selected dipoles isgoverned by the Larmor equation. That is to say, the precessionfrequency of a dipole in a magnetic field, and hence the appropriatefrequency for exciting resonance in that dipole, is a product of thegyromagnetic ratio γ of the dipole and the strength of the magneticfield. In a 1.5 T magnetic field, hydrogen (¹H) dipoles have a resonancefrequency of approximately 64 MHz. Generally in MRI, the hydrogenspecies is excited because of its abundance and because it yields astrong MR signal. As a result, typical MRI apparatus are equipped withbuilt-in whole-body RF coils tuned to the resonant frequency forhydrogen.

One obstacle to overcome in MRI is potential degradation in the imagereconstruction due a low signal to noise ratio (SNR) in the acquired MRsignals or echos.

Previously developed methods employed to address the SNR problem havehad generally limited success due to various drawbacks. Some multi-echosequences decrease the bandwidth of data collection for each echo insuccession such that the later echos, which are of a lower amplitude dueto MR signal decay, are collected at a lower bandwidth to preserve SNRas best as possible. One example is multi-echo spin echo sequences whichprovide different contrasts in a set of images built each in turn fromone of the echos. However, these techniques do not vary the bandwidth ofdata collection within a single image. Additionally, the SNR remainsdegraded by the noise accompanying data from the corners of k-spacewhich are not fully visualized in the image.

Normally, a number of rows or horizontal lines with a predeterminednumber of sample points are collected for an MR image. This raw dataoften fills k-space in a rectangular or square shape. Employment of acircular Fermi filter applied to the raw MR data to chop off the cornersof k-space has been shown to improve SNR by as much as 13%. However, thedisadvantage associated with this technique is that resources are wastedon the collection of data which is ultimately discarded.

There is also the variable encoding time (VET) method. Recognizing thatcentral lines of k-space generally employ less phase encoding ascompared to the outer lines, the VET method shortens the inter-echospacing of a sequence while varying the data sampling window. Byshortening the data sampling window time and preserving the samesampling bandwidth, time is made available for larger phase encodelobes, or the time is made available for more data samples when thedesired phase encode amounts are low. This allows for the trimming ofk-space corners.

The present invention contemplates a new and improved data collectiontechnique which overcomes the above-referenced problems and others.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, a method of MRIdata collection with multiple data acquisitions for a single imagereconstruction is provided. It includes initiating an MRI pulse sequenceand collecting MRI data from a first resulting echo at a first samplerate and bandwidth. Thereafter, the sample rate and bandwidth are variedsuch that they are set to a new sample rate and bandwidth. MRI data isthen collected from a next resulting echo at the new sample rate andbandwidth. The steps of varying the bandwidth and sample rate andcollection of resulting echos are repeated for consecutive echos until adesired amount of MRI data is collected. Ultimately, an imagerepresentation is reconstructed from the collected MRI data.

In accordance with a more limited aspect of the present invention,variations in the sample rate and bandwidth are such that a totalcollection time for each echo remains substantially constant.

In accordance with a more limited aspect of the present invention, thecollection of MRI data is conducted with a fixed duration window suchthat varying the sample rate varies a number of data points sampled.

In accordance with a more limited aspect of the present invention,collected MRI data from the first echo is mapped to a central line ofk-space.

In accordance with a more limited aspect of the present invention, latercollected MRI data from subsequent echos are progressively mapped toouter lines of k-space.

In accordance with a more limited aspect of the present invention, thebandwidth is varied such that it is progressively made narrower.

In accordance with a more limited aspect of the present invention, thenumber of data points sampled decreases as progressively outer lines ofk-space are mapped such that a circular shaped area of k-space isfilled.

In accordance with a more limited aspect of the present invention, theMRI pulse sequence is selected from a group consisting of a GSEsequence, a FSE sequence, and a single shot FSE sequence.

In accordance with a more limited aspect of the present invention, thebandwidth is varied by changing an amplitude of a read gradient so thata predetermined FOV is maintained.

In accordance with a more limited aspect of the present invention, thebandwidth selected for each echo is determined based on the echo'srelative signal strength such that lower signal strengths correspond tolower selected bandwidths.

In accordance with another aspect of the present invention, an MRIapparatus is provided. It includes a main magnet that generates asubstantially uniform temporally constant main magnetic field through anexamination region wherein an object being imaged is positioned. Amagnetic gradient generator produces magnetic gradients in the mainmagnetic field across the examination region. A transmission systemincludes an RF transmitter that drives an RF coil which is proximate tothe examination region. A sequence control manipulates the magneticgradient generator and the transmission system to produce an MRI pulsesequence. The MRI pulse sequence induces magnetic resonance echos fromthe object. A reception system includes a receiver that receives anddemodulates the echos at varying sample rates and varying bandwidths. Areconstruction processor reconstructs a single image from data collectedvia the reception system, and an output device produces a human viewablerendering of the image.

In accordance with a more limited aspect of the present invention,variations in the sample rate and variations in the bandwidth are suchthat a total collection time for each echo remains substantiallyconstant.

In accordance with a more limited aspect of the present invention,reception of each echo is conducted with a fixed duration window suchthat variations in sample rate translate to variations in number of datapoints sampled.

In accordance with a more limited aspect of the present invention,multiple acquisitions from one echo train are used together inreconstruction of one image.

In accordance with a more limited aspect of the present invention, theMRI pulse sequence is selected from a group consisting of a GSEsequence, a FSE sequence, and a single shot FSE sequence.

In accordance with a more limited aspect of the present invention, thebandwidth is varied by changing an amplitude of a read gradient appliedvia the magnetic gradient generator so that a predetermined FOV ismaintained.

In accordance with a more limited aspect of the present invention, thebandwidth selected for each echo is determined based on the echo'srelative signal strength such that lower signal strengths correspond tolower selected bandwidths.

In accordance with a more limited aspect of the present invention, thebandwidth selected for each echo is determined based on the echo'sultimate position in k-space.

One advantage of the present invention is that it improves the SNR inMRI.

Another advantage of the present invention is its efficient use of scantimes.

Still further advantages and benefits of the present invention willbecome apparent to those of ordinary skill in the art upon reading andunderstanding the following detailed description of the preferredembodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention may take form in various components and arrangements ofcomponents, and in various steps and arrangements of steps. The drawingsare only for purposes of illustrating preferred embodiments and are notto be construed as limiting the invention.

FIG. 1 is a diagrammatic illustration of an MRI apparatus in accordancewith aspects of the present invention;

FIG. 2 is a diagrammatic illustration of an MRI pulse sequence inaccordance with aspects of the present invention; and,

FIG. 3 is a diagrammatic illustration of k-space in accordance withaspects of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference to FIG. 1, a main magnetic field control 10 controlssuperconducting or resistive magnets 12 such that a substantiallyuniform temporally constant main magnetic field, B₀, is created along az axis through an examination region 14. A couch (not illustrated)suspends and/or positions an object to be examined, such as a patient orphantom, within the examination region 14. A magnetic resonance echomeans applies a series of RF and magnetic field gradient pulses toinvert or excite magnetic spins, induce magnetic resonance, refocusmagnetic resonance, manipulate magnetic resonance, spatially andotherwise encode the magnetic resonance, to saturate spins, and the liketo generate magnetic resonance imaging sequences. More specifically,gradient pulse amplifiers 20 apply current pulses to selected ones orpairs of gradient coil assemblies 22 to create magnetic field gradientsalong x, y, and z axes of the examination region 14. A digital RFtransmitter 24 drives a whole-body RF coil 26 to transmit RF pulses orpulse packets into the examination region 14. A typical RF pulse iscomposed of a packet of immediately contiguous pulse segments of shortduration, which taken together with each other and any appliedgradients, achieve a selected magnetic resonance manipulation. Forwhole-body applications, the resonance signals or echos are commonlypicked up by the whole-body RF coil 26.

For generating images of local regions of the subject, specialized RFcoils are placed contiguous to the selected region of interest. Forexample, an insertable RF coil may be inserted surrounding a selectedregion at the isocenter of the bore. The insertable RF coil is used toexcite magnetic resonance and receive magnetic resonance signalsemitting from the patient in the region being examined. Alternatively,the insertable RF coil can be used to only receive resonance signalsintroduced by the wholebody RF coil 26. In any event, the resultant RFsignals are picked up by the whole-body RF coil 26, the insertable RFcoil, or other specialized RF coil and demodulated by a receiver 30,preferably a digital receiver.

A sequence control circuit 40 controls the gradient pulse amplifiers 20and the transmitter 24 to generate any of a plurality of multiple echosequences such as gradient and spin echo (GSE) imaging, fast spin echo(FSE) imaging, single shot FSE imaging, and the like. For the selectedsequence, the receiver 30 receives a plurality of data lines in rapidsuccession following each RF excitation pulse. Ultimately, the RFsignals are received, demodulated, and reconstructed into an imagerepresentation by a reconstruction processor 50 which applies atwo-dimensional (2D) Fourier transform or other appropriatereconstruction algorithm. The image may represent a planar slice throughthe patient, an array of parallel planar slices, a three-dimensionalvolume, or the like. The image is then stored in an image memory 52where it is accessed by a display, such as a video monitor 54 or otherhuman viewable display or output device that provides a rendering of theresultant image.

While the invention herein is described with reference to the MRIapparatus detailed above, it is appreciated that the invention isapplicable to other MRI apparatus. For example, the invention is equallyamenable to open geometry magnets wherein opposing pole pieces, joinedby a ferrous flux return path, define an examination regiontherebetween.

In any event, with reference to FIG. 2 and continuing reference to FIG.1, the sequence control 40 initiates and directs an MRI pulse sequencethat generates the signal received by the receiver 30. For purposes ofillustration herein, the MRI pulse sequence is assumed to be a FSEsequence. However, a single shot FSE sequence, a GSE sequence, or thelike may also be employed. For each image reconstructed, during theapplication of separate slice select gradients 60 to the gradient coilassembly 22, a series of RF pulses are applied to the RF coil 26 via theRF transmitter 24. In the exemplary FSE sequence, the RF pulses includean initial 90° flip angle resonance exciting RF pulse 62 followed by aseries of 180° flip angle refocusing RF pulses 64. The application ofeach refocusing pulse 64 results in an echo 66. Collectively the echos66 make up the echo train of the signal. Each echo 66 is phase encodedby applying, to the gradient coil assembly 22, a phase encode gradientpulse 68 preceding and following each echo 66. The individual echos 66are separately phase encoded by varying the amplitude and/or duration ofthe phase encoding gradient pulse 68. Each echo 66 is then collectedunder a readout gradient pulse 70 applied to the gradient coil assembly22. The profile of the readout gradient pulses 70 includes a centralamplitude that varies between echos 66, and extra lobes at either end.This profile is constructed so that the echos 66 are collected underreadout gradients of different amplitudes while the total area undereach readout gradient pulse 70 is kept the same between each refocusingRF pulse 64. In any event, the receiver 30 collects and/or samples theechos 66 under a fix duration sampling window 72.

With reference to FIG. 3 and continuing reference to FIGS. 1 and 2, aseach echo 66 is collected and/or sampled by the receiver 30, the raw MRdata is loaded into a memory matrix otherwise known as k-space 80. It isthis raw data in k-space 80 that is the object of the 2D Fouriertransform or other appropriate reconstruction algorithm carried out bythe reconstruction processor 50. Each of the collected echos 66 ismapped into its own corresponding row or horizontal line of k-space 80such that each successively sampled data value occupies successivecolumns. The row each echo 66 is assigned to is based on the particularphase encoding imparted thereto via the phase encoding gradient pulses68. Optionally, in a preferred embodiment, a first collected echo 66 ismapped to a central line of k-space 80 while successively collectedechos 66 are mapped to further out neighboring lines of k-space 80 in aprogressive manner.

The collected echos 66 have variable bandwidths and are sampled at avariable sampling rate. The variable bandwidths are achieved byregulating the amplitude of the readout gradient pulses 70 applied tothe gradient coil assembly 22 such that a desired field of view (FOV) ismaintained for each echo 66. In the illustrated pulse sequence, theechos 66 are sampled at progressively narrower bandwidths under readoutgradient pulses 70 having progressively smaller amplitudes during echocollection. In this manner, the later echos 70 in the echo train whichtypically have lower signal strengths are sampled at the lowerbandwidths thereby improving their SNR.

Additionally, the echos 66 are collected at variable sampling rates.Being that the sampling window 72 has a fixed duration, the result is avariable number of data samples for each of the echos 66. In a preferredembodiment, the sampling rate (and hence the number of data pointssampled) is varied such that as successive lines of k-space 80 arefilled, the filled area of k-space 80 takes a circular shape. That is tosay, the relationship between the phase encoding imparted by the phaseencoding gradient pulses 68 and the sampling rate is such that acircular area of k-space 80 is filled with sampled data values. Toachieve the circular shape, phase encoded lines assigned more toward acenter of k-space 80 have a relatively higher sampling rate such thatmore data points are collected within the fixed duration sampling window72. The relatively higher number data samples in turn results inrelatively longer data lines.

As the data lines grow progressively shorter toward the edges of k-space80, the filled portion of k-space 80 take on the shape of a circle. Inthis manner, similar to the application of a Fermi filter, the SNR isimproved by eliminating the corners of k-space 80 which are not fullyvisualized in the image reconstruction. Alternately, as opposed to acircle, other desired shapes are employed such as, for example, anelliptical shape.

The invention has been described with reference to the preferredembodiments. Obviously, modifications and alterations will occur toothers upon reading and understanding the preceding detaileddescription. It is intended that the invention be construed as includingall such modifications and alterations insofar as they come within thescope of the appended claims or the equivalents thereof.

Having thus described the preferred embodiments, the invention is nowclaimed to be:
 1. A method of magnetic resonance imaging comprising: (a)initiating an MRI pulse sequence resulting in a plurality of echos; (b)collecting MRI data from one of the plurality of echos at a first samplerate and bandwidth; (c) varying the sample rate and bandwidth such thatthey are set to a new sample rate and bandwidth while maintainingsubstantially the same collection time; (d) collecting MRI data fromanother of the plurality of echos at the new sample rate and bandwidth;(e) repeating steps (c) and (d) for the plurality of echos until adesired amount of MRI data is collected; and, (f) reconstructing asingle image representation from the MRI data collected at varyingsample rates and bandwidths.
 2. A method of MRI data collectioncomprising: (a) initiating an MRI pulse sequence resulting in aplurality of echos; (b) collecting MRI data from one of the plurality ofechos at a first sample rate and bandwidth; (c) varying the sample rateand bandwidth so that they are set to a new sample rate and bandwidth,wherein a total collection time for each echo remains substantiallyconstant; (d) collecting MRI data from another of the plurality of echosat the new sample rate and bandwidth; (e) repeating steps (c) and (d)until a desired amount of MRI data is collected; and, (f) reconstructingan image representation from the collected MRI data.
 3. The methodaccording to claim 2, wherein the collection of MRI data is conductedwith a fixed duration window such that varying the sample rate varies anumber of data points sampled.
 4. The method according to claim 3,wherein collected MRI data from a first of the plurality of echos ismapped to a central line of k-space.
 5. The method according to claim 4,wherein later collected MRI data from echos subsequent to the first areprogressively mapped to outer lines of k-space.
 6. The method accordingto claim 5, wherein the bandwidth is varied such that it isprogressively made narrower.
 7. The method according to claim 6, whereinthe number of data points sampled decreases as progressively outer linesof k-space are mapped such that a circular shaped area of k-space isfilled.
 8. The method according to claim 1, wherein the MRI pulsesequence is selected from a group consisting of a GSE sequence, a FSEsequence, and a single shot FSE sequence.
 9. The method according toclaim 1, wherein the bandwidth is varied by changing an amplitude of aread gradient so that a predetermined FOV is maintained.
 10. The methodaccording to claim 1, wherein the bandwidth selected for each echo isdetermined based on the echo's relative signal strength such that lowersignal strengths correspond to lower selected bandwidths.
 11. An MRIapparatus comprising: a main magnet that generates a substantiallyuniform temporally constant main magnetic field through an examinationregion wherein an object being imaged is positioned; a magnetic gradientgenerator that produces magnetic gradients in the main magnetic fieldacross the examination region; a transmission system which includes anRF transmitter that drives an RF coil which is proximate to theexamination region; a sequence control which manipulates the magneticgradient generator and the transmission system to produce an MRI pulsesequence, said MRI pulse sequence inducing magnetic resonance echos fromthe object, said echos having varying bandwidths; a reception systemwhich includes a receiver that receives and demodulates the echos suchthat a varying number of data points are collected for each echo; areconstruction processor that reconstructs a single image using datacollected via the reception system from echos having varying bandwidths;and, an output device that produces a human viewable rendering of theimage.
 12. An MRI apparatus which induces magnetic resonance echos froman object being imaged, said MRI apparatus comprising: a receptionsystem which includes a receiver that receives and demodulates the echosat varying sample rates and varying bandwidths, wherein a totalcollection time for each echo remains substantially constant; and, areconstruction processor that reconstructs a single image from echoshaving varying sample rates and varying bandwidths.
 13. An MRI apparatuswhich induces magnetic resonance echos from an object being imaged, saidMRI apparatus comprising: a reception system which includes a receiverthat receives and demodulates the echos at varying sample rates andvarying bandwidths, wherein reception of each echo is conducted with afixed duration window such that variations in sample rate translate tovariations in number of data points sampled; and, a reconstructionprocessor that reconstructs a single image from echos having varyingsample rates and varying bandwidth.
 14. The MRI apparatus according toclaim 11, wherein multiple acquisitions from one echo train are usedtogether in reconstruction of one image.
 15. The MRI apparatus accordingto claim 14, wherein the MRI pulse sequence is selected from a groupconsisting of a GSE sequence, a FSE sequence, and a single shot FSEsequence.
 16. The MRI apparatus according to claim 11, wherein thebandwidth is varied by changing an amplitude of a read gradient appliedvia the magnetic gradient generator so that a predetermined FOV ismaintained.
 17. The MRI apparatus according to claim 11, wherein thebandwidth selected for each echo is determined based on the echo'srelative signal strength such that lower signal strengths correspond tolower selected bandwidths.
 18. The MRI apparatus according to claim 11,wherein the bandwidth selected for each echo is determined based on theecho's ultimate position in k-space.